The present invention relates generally to electrical unidirectional current devices, and more specifically to a device which is suitable for use as a model of a unidirectional mechanical valve.
Installation or modification of natural gas or other fluid distribution systems requires consideration of a number of factors before work is undertaken. Variations in loads, distribution paths, pipe sizes and compressor speeds all have effects on the operation of the system as a whole. Compression waves created in the gas by the operation of reciprocating pumps and compressors are especially troublesome as resonances can be set up in the system. Acoustical resonance increases metal fatigue and shortens the life of joints, valves and other components of the system.
To assist in planning for the control of undesirable pulsations and vibrations in a system, an electrical analog of all fluid transfer components can be created. Present electrical systems analogize current to mass flow of the gas and voltage to pressure. Inductors, capacitors and resistors are used to model the mechanical properties of pipes and other components in the distribution system. A detailed model of a distribution system or subsystem can be set up and studied to predict the effects caused by changing various parameters in the operation of the system. Examples of the use of gas pumping systems analogs are found in U.S. Pat. Nos. 2,951,638 and 2,979,940.
Reciprocating pumps and compressors utilize unidirectional mechanical valves at the intake and discharge ports of the cylinder. These valves will open and allow gas to flow when the fluid pressure at the upstream end is greater than that at the downstream end. No fluid will flow when the pressure at the downstream end of the valves is greater than that at the upstream end. In a capacitor charge pump model of a reciprocating compressor, voltage is utilized as the electrical analog of fluid pressure. Semiconductor diodes are presently utilized as the electrical analogs of unidirectional mechanical valves. An ideal diode would only conduct current from the anode to the cathode, and only when voltage is greater than the voltage at the cathode.
The properties of actual diodes differ from that of the ideal diode, and their use in such circuits has several major drawbacks. One drawback is the presence of a voltage drop across the diode when it is conducting current. This voltage drop can cause the diode to present a substantial effective resistance when conducting small currents. For example, with a typical voltage drop of 0.7 volts, a diode conducting 0.1 mA presents an effective resistance to current flow of 7,000 ohms. Another important drawback of semiconductor diodes is that the diode will not begin to conduct current until the voltage difference between the anode and the cathode reaches the turn-on voltage, which is typically around 0.5 volts. A diode wil not switch to the conducting state and begin to conduct current until this threshold difference is reached. Yet another drawback is that a diode does not change from the nonconducting to the conducting state immediately, but rather has a finite switching speed which depends in part on the rate of change of the voltage across the diode.
The operation of a diode is adversely affected by the junction capacitance between the anode and the cathode. Further, the effective resistance of the diode does not behave in a manner analogous to the resistance to fluid flow presented by a mechanical valve. In a mechanical valve, increasing fluid flow causes increasing resistance. Contrariwise, higher current through a semiconductor diode gives a lower effective resistance.